Fuse clips, fuse clamps, bus bars, heat sinks, and other metal objects to which the terminals of electric fuses are secured are usually cooler than the weak spots of those electric fuses. Consequently, it is traditional to expect, and to count on, a gradient between the temperatures of the weak spot and of the terminals of an electric fuse; and that gradient fosters cooling of that weak spot by causing heat to flow to those relatively-cool terminals. However, because the fusible elements of current-limiting fuses are made thin, those fusible elements limit the rate at which heat can be caused to flow from the weak spots to the terminals of such electric fuses.
Some electric fuses have fusible elements which perform only one function, whereas other electric fuses have fusible elements which perform dual functions. One example of an electric fuse that has a fusible element which performs only one function is an electric fuse that is connected in series relation with a circuit breaker; and the only function to be performed by the fusible element of such an electric fuse is to open the circuit on a heavy overload or short circuit. Another example of an electric fuse that has a fusible element which performs only one function is an electric fuse which has a spring-biased connector or a large mass of solder that can respond to a prolonged low overload to open the circuit; and the only function to be performed by the fusible element of such an electric fuse is to open the circuit on a heavy overload or short circuit. One example of an electric fuse that has a fusible element which performs a dual function is a renewable electric fuse that has a fusible element which is able to open the circuit in response to a prolonged low overload or to a heavy overload or short circuit. Another example of an electric fuse that has a fusible element which performs a dual function is an electric fuse that has a silver or copper fusible element with a mass of tin riveted or bonded to it. An electric fuse which has a spring-biased connector or a large mass of solder that can respond to a prolonged low overload to open the circuit is referred to as a dual-element electric fuse; and, similary, an electric fuse that has a silver or copper fusible element with a mass of tin riveted or bonded to it is referred to as a dual-element electric fuse.
Summary of the Invention: The present invention provides a current-limiting, single-element, dual-function, electric fuse which has a fusible element with at least one weak spot; and that weak spot is closer to the inner surface of the housing of that electric fuse than it is to either terminal of that electric fuse. That weak spot is directly contacted by arc-quenching filler which also directly contacts, and which can transfer heat to, that inner surface. The major portion of the outer surface of the housing of the electric fuse is exposed; and that housing is made from a material which has a thermal conductivity in excess of thirty thousandths of a calorie per square centimeter of cross section per centimeter of length per second of time per degree centigrade. As a result, that arc-quenching material and that housing can rapidly dissipate heat which is generated by the weak spot, and can thereby enable that weak spot to be made with a very small cross section. That weak spot will have a temperature in excess of eighty percent of the melting temperature of the material of that fusible element when the electric fuse is operating at its maximum continuous current carrying capacity; and hence that weak spot can rapidly respond to a low but potentially-harmful overload to open the circuit. It is, therefore, an object of the present invention to provide a current-limiting, single-element, dual-function, electric fuse which has a fusible element with at least one weak spot of very small cross section, and which has that weak spot operating at temperatures in excess of eighty percent of the melting temperature of that fusible element when the electric fuse is operating at its maximum continuous current carrying capacity.
The present invention disposes the weak spot of the electric fuse so the radial distance between that weak spot and the adjacent inner surface of the housing for that electric fuse is less than 64 millimeters, and so the axial distance between that weak spot and either terminal of that electric fuse is greater than eighty millimeters. Further, the present invention disposes the arc-quenching filler in direct contact with the weak spot and with the inner surface of the housing for the electric fuse. As a result, appreciable amounts of the heat which is generated by the small cross section weak spot will pass to, and be radiated by, the exposed outer surface of the housing. It is, therefore, an object of the present invention to dispose the weak spot of a current-limiting, single-element, dual-function, electric fuse so the radial distance between the weak spot and the adjacent inner surface of the housing for that electric fuse is less than 64 millimeters and so the axial distance between that weak spot and either terminal of that electric fuse is greater than 80 millimeters.
Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.
In the drawing and accompanying description several preferred embodiments of the present invention are shown and described but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims.